From b2b109041ecd1095384f5be5bb9badd13c1cf286 Mon Sep 17 00:00:00 2001 From: Jean-Louis Dupond Date: Tue, 3 Oct 2023 14:52:37 +0200 Subject: qcow2: keep reference on zeroize with discard-no-unref enabled When the discard-no-unref flag is enabled, we keep the reference for normal discard requests. But when a discard is executed on a snapshot/qcow2 image with backing, the discards are saved as zero clusters in the snapshot image. When committing the snapshot to the backing file, not discard_in_l2_slice is called but zero_in_l2_slice. Which did not had any logic to keep the reference when discard-no-unref is enabled. Therefor we add logic in the zero_in_l2_slice call to keep the reference on commit. Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1621 Signed-off-by: Jean-Louis Dupond Message-Id: <20231003125236.216473-2-jean-louis@dupond.be> [hreitz: Made the documentation change more verbose, as discussed on-list] Signed-off-by: Hanna Czenczek --- qapi/block-core.json | 24 ++++++++++++++---------- 1 file changed, 14 insertions(+), 10 deletions(-) (limited to 'qapi') diff --git a/qapi/block-core.json b/qapi/block-core.json index 9996125..ca390c5 100644 --- a/qapi/block-core.json +++ b/qapi/block-core.json @@ -3528,16 +3528,20 @@ # @pass-discard-other: whether discard requests for the data source # should be issued on other occasions where a cluster gets freed # -# @discard-no-unref: when enabled, discards from the guest will not -# cause cluster allocations to be relinquished. This prevents -# qcow2 fragmentation that would be caused by such discards. -# Besides potential performance degradation, such fragmentation -# can lead to increased allocation of clusters past the end of the -# image file, resulting in image files whose file length can grow -# much larger than their guest disk size would suggest. If image -# file length is of concern (e.g. when storing qcow2 images -# directly on block devices), you should consider enabling this -# option. (since 8.1) +# @discard-no-unref: when enabled, data clusters will remain +# preallocated when they are no longer used, e.g. because they are +# discarded or converted to zero clusters. As usual, whether the +# old data is discarded or kept on the protocol level (i.e. in the +# image file) depends on the setting of the pass-discard-request +# option. Keeping the clusters preallocated prevents qcow2 +# fragmentation that would otherwise be caused by freeing and +# re-allocating them later. Besides potential performance +# degradation, such fragmentation can lead to increased allocation +# of clusters past the end of the image file, resulting in image +# files whose file length can grow much larger than their guest disk +# size would suggest. If image file length is of concern (e.g. when +# storing qcow2 images directly on block devices), you should +# consider enabling this option. (since 8.1) # # @overlap-check: which overlap checks to perform for writes to the # image, defaults to 'cached' (since 2.2) -- cgit v1.1